The present invention relates to a technique that prints an image with a head assembly that is obtained by combining a plurality of nozzle units, from which ink droplets are ejected.
The printing apparatus that has a plurality of nozzles formed in a print head and causes ink droplets to be ejected from the respective nozzles so as to create ink dots on a printing medium and thereby complete a printed image has widely been used as an output device of various image-related equipment. The printing apparatus prints the image while repeatedly carrying out main scan and sub-scan of the print head as discussed below. This prior art technique carries out one pass of the main scan of the print head in a direction crossing an extending direction of a nozzle array to form plural arrays of ink dots (hereinafter referred to as raster lines). After a pass of the sub-scan in a direction crossing an extending direction of the raster lines, the technique carries out another pass of the main scan to form another plural raster lines at different positions. The repeated cycles of the main scan and the sub-scan of the print head form raster lines at all the possible positions on the printing medium, thereby completing a desired image on the printing medium.
In this printing apparatus, the greater number of nozzles formed in the print head effectively increases the number of raster lines formed by each pass of the main scan and thereby enhances the printing speed. The effect of the large-sized print head with the greater number of nozzles is especially significant in the case of printing on large sheets of paper like the A3 and A0 sizes, since a large number of raster lines should be formed in such large sheets of paper.
It is thought convenient to construct a head assembly by combining a plurality of nozzle units of conventionally used print heads, instead of newly manufacturing a large-sized integral print head. This method enables a head assembly of a desired size to be constructed by combining an appropriate number of nozzle units. Application of the manufacturing technique of the conventional print heads facilitates the manufacture of the head assembly, compared with the manufacture of the large-sized integral print head.
The respective nozzle units combined to construct the large-sized head assembly generally have a slight difference in ink ejection characteristics. Such variation may deteriorate the printing quality in the case of printing an image with the head assembly. Even a small error in assembling position of each nozzle unit causes the positions of all the dots created by the nozzle unit to be deviated uniformly by a fixed amount. The difference in error of the assembling position among the respective nozzle units may accordingly deteriorate the printing quality at positions corresponding to joints of adjoining nozzle units.
The object of the present invention is thus to provide a technique that prevents the printing quality from deteriorating at positions corresponding to joints of adjoining nozzle units in the case of printing an image with a large-sized print head obtained by combining a plurality of nozzle units.
At least part of the above and the other related objects is attained by a printing apparatus that causes ink droplets to be ejected from a nozzle array, which is formed by arranging a plurality of nozzles, so as to create ink dots on a printing medium and thereby print an image. The printing apparatus includes: a head assembly obtained by combining a plurality of nozzle units arranged in an extending direction of the nozzle array, that is, in a sub-scanning direction, where each nozzle unit has the plurality of nozzles; a raster formation unit that creates the ink dots while moving the head assembly in a main scanning direction, which crosses the sub-scanning direction, so as to form raster lines on the printing medium at intervals of a k raster lines space, where each raster line represents an array of dots; and a sub-scan unit that moves the head assembly in the sub-scanning direction, which crosses the main scanning direction, by a predetermined amount. The sub-scan unit carries out respective passes of sub-scan of the head assembly, in order to record all raster lines included in an effective area on the printing medium, each pass of the sub-scan causing a rear most nozzle included in each nozzle unit to be located at a specific position that is apart by at least k raster lines from a certain position where a rear most nozzle included in another nozzle unit is located prior to the pass of the sub-scan.
There is also a method corresponding to the printing apparatus of the above arrangement. The present invention is accordingly directed to a method of printing an image by causing ink droplets to be ejected from a nozzle array, which is formed by arranging a plurality of nozzles, to create ink dots on a printing medium while changing a relative position of the nozzle array to the printing medium. The method includes the steps of: creating ink dots while successively moving a head assembly in a direction that crosses an extending direction of the nozzle array, so as to form raster lines on the printing medium at intervals of a k raster lines space, where each raster line represents an array of dots and the head assembly is obtained by combining a plurality of nozzle units arranged in the extending direction of the nozzle array, each nozzle unit having the plurality of nozzles; and successively moving the head assembly in the extending direction of the nozzle array by a predetermined amount, so as to record all raster lines included in an effective area on the printing medium, each movement of the head assembly causing a rear most nozzle included in each nozzle unit to be located at a specific position that is apart by at least k raster lines from a certain position where a rear most nozzle included in another nozzle unit is located prior to the movement of the head assembly.
In the printing apparatus and the corresponding method of the present invention, the head assembly is subjected to each pass of the main scan that causes ink droplets to be ejected from the nozzle array, which is an alignment of the plurality of nozzle units, and thereby creates ink dots on the printing medium. This forms a plurality of raster lines at equal intervals of a k raster lines space on the printing medium. A next pass of the main scan is carried out after one pass of the sub-scan of the head assembly. The technique successively carries out the main scan and the sub-scan of the head assembly and completes a printed image on the printing medium. Here k is a natural number of not less than 1. Each pass of the sub-scan of the head assembly is carried out to cause a rear most nozzle included in each nozzle unit of the head assembly, that is, a nozzle at the opposite end in the sub-scanning direction, to be located at a specific position that is apart by at least k raster lines from a certain position where a rear most nozzle included in another nozzle unit is located prior to the pass of the sub-scan.
This arrangement causes any position printed by the joint of nozzle units in each pass of the main scan to be printed by a part of the nozzle unit other than the joint in another pass of the main scan. This effectively prevents any overlap of the printing positions corresponding to the joints of the nozzle units. Even if some difference in printing-related characteristic between the nozzle units slightly worsens the picture quality at the joint of the nozzle units, this arrangement effectively prevents the picture quality-deteriorating factors from being collectively accumulated in any specific part on a resulting printed image and thereby makes the deteriorating picture quality sufficiently inconspicuous. This arrangement further relieves the deteriorating picture quality at the joint in the course of printing and eventually ensures the good picture quality.
In order to clearly show the effects of the present invention, the following description first regards the reason for the deteriorating picture quality in the case of sub-scan that causes overlaps of printing positions corresponding to joints of nozzle units and then an arrangement of sub-scan that avoids any overlap of the printing positions corresponding to the joints of the nozzle units and thereby prevents deterioration of the printing quality. The description subsequently regards the conditions for the sub-scan that avoids any overlap of the printing positions corresponding to the joints of the nozzle unit.
FIG. 15 conceptually shows the reason for the deteriorating picture quality in the case of sub-scan that causes overlaps of printing positions corresponding to joints of nozzle units when an image is printed with a head assembly including a plurality of nozzle units.
The conceptual view of FIG. 15 shows a process of printing an image by main scan and sub-scan of a head assembly, which includes three nozzle units A, B, and C and functions like an integral print head. In the example of FIG. 15, the nozzle units A, B, and C have an identical nozzle arrangement, and each pass of the sub-scan shifts the head assembly by ⅓ of the whole length of the head assembly. In order to record all possible raster lines on the printing medium by the sub-scan of a nozzle array, the nozzle array and the amount of sub-scan should hold a predetermined relationship, which will be discussed later. In the specification hereof, to appropriately select the amount of sub-scan for the nozzle array and enable formation of all possible raster lines on the printing medium is referred to as xe2x80x98to complete the interlacexe2x80x99.
The following describes the meaning of FIG. 15. In the first pass of the main scan of the head assembly, the three nozzle units A, B, and C included in the head assembly are respectively located at positions A1, B1, and C1 to form raster lines. The first pass of the sub-scan of the head assembly by ⅓ of the whole length of the head assembly respectively moves the nozzle units A, B, and C to positions A2, B2, and C2 to form raster lines. The next pass of the sub-scan causes the nozzle units A, B, and C to respectively form raster lines at positions A3, B3, and C3. The procedure repeatedly carries out the main scan and the sub-scan of the head assembly to form all the possible raster lines on the printing medium and thereby complete a desired printed image on the printing medium.
The three nozzle units A, B, and C form a continuous nozzle array, but the individual difference among the nozzle units subtly varies the ejection characteristic of the ink droplets and may the assembled construction undesirably recognized. There may also be attachment error. For example, it is assumed that the actual attachment position of the nozzle unit B to the nozzle unit A is farther by dL from its designed attachment position; that is, the attachment position of the nozzle unit B to the nozzle unit C is closer y dL from its designed attachment position. This causes the raster lines formed by the nozzle unit B to be uniformly deviated by dL. In this case. the boundary between the nozzle units A and B and the boundary between the nozzle units B and C are made undesirably conspicuous.
In the example of FIG. 15, at the position defined by the arrow P12, the boundary between the nozzle units B and C in the first pass of the main scan overlaps with the boundary between the nozzle units A and B in the second pass of the main scan. As described previously, the position printed by the joint of the nozzle units has the deteriorating picture quality, due to the individual difference between the nozzle units and the attachment error of the nozzle unit. The positions printed by the joints of the nozzle units in an overlapping manner like the position of the arrow P12 may be undesirably conspicuous and thus worsen the printing quality. The deteriorating picture quality due to the conspicuous printing position corresponding to the joint of the nozzle units may be found not only at the position of the arrow P12 but at any other positions, for example, at the positions defined by the arrows P23 and P34 in the example of FIG. 15.
When the amount of sub-scan of the head assembly has some error, the printing position corresponding to the rear most end of the head assembly is made undesirably conspicuous to worsen the picture quality. This phenomenon is described with regard to the position P12 in the example of FIG. 15. Raster lines are formed at equal intervals in both the areas A2 and A3 by the identical nozzle unit A. It is accordingly thought that the intervals between any adjoining raster lines are identical in these two areas. In the case where the amount of sub-scan of the head assembly has some error, for example, an excess of dS, there is a region having the wider interval between adjoining raster lines by dS on the boundary between the areas A2 and A3. In this region, the boundary between the area printed in the second pass of the main scan (that is, the area A2) and the area printed in the third pass of the main scan (that is, the area A3) is made undesirably conspicuous. When the error is an insufficiency of dS, on the other hand, there is a region having the narrower interval between adjoining raster lines by dS on the boundary between the areas A2 and A3. In this region, the boundary between the two areas is also made conspicuous. When the amount of sub-scan has some error, there is a discontinuous area having the varied interval between adjoining raster lines at the rear most end of the head assembly. Such discontinuousness often worsens the picture quality.
In the example shown in FIG. 15, the picture quality-deteriorating factor due to the individual difference between the adjoining nozzle units and the attachment error arises at identical positions where the picture quality-deteriorating factor due to the error in amount of sub-scan of the nozzle unit arises. The printing quality may thus be significantly worsened at such positions. For example, at the position defined by the arrow P12, the picture quality-deteriorating factor arising at the joint of nozzle units A2 and B2 is combined with the picture quality-deteriorating factor arising on the boundary between the nozzle units A2 and A3. The picture quality may thus be significantly worsened at the position of the arrow P12. In the actual design, in order to prevent deterioration of the picture quality due to the individual factors, the individual difference between the adjoining nozzle units, the attachment error, and the error in amount of sub-scan are limited to low levels. Accumulation of these factors, however, naturally causes deterioration of the printing quality. This happens not only at the position of the arrow P12 but at other positions, for example, those defined by the arrows P23 and P34. The sub-scan that makes the printing positions corresponding to the joints of the nozzle units overlap each other may worsen the printing quality.
FIG. 16 shows one example of the sub-scan that avoids any overlap of the printing positions corresponding to the joints of the nozzle units. The total number of nozzles and the interval between the nozzles are identical with those of the example shown in FIG. 15. The same value is also set to the amount of sub-scan. The difference from the example of FIG. 15 is that the position of the joint of the adjoining nozzle units is changed to prevent the printing positions corresponding to the joints of any nozzle units from overlapping each other in the course of the sub-scan of the head assembly. In the example of FIG. 16, the number of nozzle units included in the head assembly is changed from 3 to 2 with a view to changing the position of the joint of the nozzle unit. In another possible procedure, the number of nozzles included in each nozzle unit may be varied among the nozzle units A, B, and C.
In the example of FIG. 16, the printing position defined by the arrow Q1 corresponds to the joint of the nozzle units A and B. There is accordingly the picture quality-deteriorating factor due to the individual difference between the two nozzle units and the attachment error of the nozzle unit. Unlike the example of FIG. 15, however, another picture quality-deteriorating factor does not appear in combination at this position. It is accordingly possible to prevent the picture quality from being significantly worsened at this position by reducing the individual difference between the nozzle units and the attachment error.
At the position defined by the arrow R23 in the example of FIG. 16, there is the picture quality-deteriorating factor due to the error in amount of sub-scan of the head assembly. Another picture quality-deteriorating factor, however, does not appear in combination at this position. It is accordingly possible to prevent the picture quality from being significantly worsened at this position by reducing the error in amount of sub-scan.
In the example of FIG. 16, the two picture quality-deteriorating factors do not arise in combination at an identical position. It is accordingly possible to prevent deterioration of the picture quality by reducing the individual difference between nozzle units, the attachment error, and the error in feeding amount of sub-scan to the extent that the individual factors alone do not worsen the picture quality.
As described above, the appropriate sub-scan of the head assembly to avoid any overlap of the printing positions corresponding to the joints of the adjoining nozzle units included in the head assembly effectively prevents deterioration of the picture quality at such printing positions. The following describes the conditions to be fulfilled for the appropriate sub-scan that avoids any overlap of the printing positions corresponding to the joints of the nozzle units.
FIG. 17 is an enlarged view showing the joint between nozzle units A and B in an n-th pass of the main scan and the joint between nozzle units C and D in an m-th pass of the main scan. In the illustration of FIG. 17, the nozzles actually present on the nozzle unit are shown by the thick solid lines. The phantom nozzles between the actually existing nozzles are shown by the thin dotted lines with a view to clearly showing that the respective nozzles are aligned at equal intervals. In the example of FIG. 17, the respective nozzles are aligned at equal intervals of a four nozzles space. The nozzle interval between adjoining nozzle units (between the nozzle units A and B or between the nozzle units C and D) is also equal to the four nozzles space.
There is the picture quality-deteriorating factor due to the individual difference between the adjoining nozzle units and the error in positioning of the nozzle unit at the joint of the nozzle units A and B (that is, on a boundary IAB between a nozzle unit An and a nozzle unit Bn in the drawing) in the n-th pass of the main scan. In a similar manner, there is the picture quality-deteriorating factor due to, for example, the individual difference between the adjoining nozzle units at the joint of the nozzle units C and D (that is, on a boundary ICD in the drawing) in the m-th pass of the main scan. Under the conditions shown in FIG. 17(a), the two picture quality-deteriorating factors arise in an overlapping manner in an area defined by IAB+ICD. The picture quality may thus significantly deteriorate in this area.
In the example of FIG. 17(b), the amount of sub-scan of the head assembly is increased by a two nozzles space from the example of FIG. 17(a). As clearly understood from the illustration, in the case of FIG. 17(b), the picture quality-deteriorating factor arising at the joint between the nozzle units A and B is not combined with the picture quality-deteriorating factor arising at the joint between the nozzle units C and D at any position. It is accordingly possible to prevent deterioration of the picture quality by reducing the individual difference between nozzle units and the attachment error to the extent that the individual factors alone do not worsen the picture quality.
In the example of FIG. 17(c), the amount of sub-scan of the head assembly is decreased by an eight nozzles space from the example of FIG. 17(a). In the case of FIG. 17(c), the picture quality-deteriorating factor arising at the joint between the nozzle units A and B is not combined with the picture quality-deteriorating factor arising at the joint between the nozzle units C and D at any position. It is accordingly possible to prevent deterioration of the picture quality by reducing the individual difference between nozzle units and the attachment error.
The attention is here paid to the rear most nozzle in each nozzle unit (for example, the nozzle at the rear most end of either the nozzle unit B or the nozzle unit D shown by the closed circle) in FIGS. 17(a) through 17(c). The following condition should be satisfied for the sub-scan that avoids any overlap of the printing positions corresponding to the joints of the nozzle units. The required condition of the sub-scan is to make the current position of the rear most nozzle in one nozzle unit (for example, the nozzle at the rear most end of the nozzle unit D shown by the closed circle) apart by at least the nozzle pitch k from a certain position where the rear most nozzle in another nozzle unit (for example, the nozzle at the rear most end of the nozzle unit B shown by the closed circle) is located previously. This arrangement effectively prevents any overlap of the printing positions corresponding to the joints of the nozzle units.
The substantially similar discussion is applicable for the picture quality-deteriorating factor due to the error in amount of sub-scan of the head assembly. When the amount of sub-scan has some error, the interval between newly formed raster lines is slightly deviated from the interval between existing raster lines formed prior to the sub-scan. This results in discontinuousness of the part adjacent to the rear most nozzle in the head assembly and makes the joint of the nozzle units undesirably conspicuous in this part. This phenomenon is described below with reference to the example of FIG. 18. FIG. 18 is an enlarged view showing the positional relationship between the joint of nozzle units A and B in an n-th pass of the main scan and the rear most end of the nozzle unit A in an (n+1)-th pass of the main scan. When the amount of sub-scan has some error, the raster line formed by the rear most nozzle in the nozzle unit A in the (n+1)-th pass of the main scan is not located at the expected position. The interval from the adjoining raster line formed in the n-th pass is not an integral multiple of the width of the raster line. This may give the discontinuous impression and thus deteriorate the picture quality in an area adjacent to the rear most end of the nozzle unit A (an area Ea in the drawing). The picture quality-deteriorating factor arising at the joint between the nozzle units A and B is also present in this area. The combination of the two picture quality-deteriorating factors makes the deteriorating picture quality recognizable. It is accordingly required to make the position of the rear most nozzle in the nozzle unit A apart by at least the nozzle pitch k from the position where the rear most nozzle in another nozzle unit is located previously. This arrangement favorably prevents the picture quality from being worsened by the combination of the two picture quality-deteriorating factors.
In the printing apparatus and the corresponding method of the present invention, the sub-scan of the head assembly is carried out to record all the raster lines included in the effective area on the printing medium and to make the rear most nozzle in each nozzle unit apart by at least k raster lines from the certain position where the rear most nozzle in another nozzle unit is located prior to the sub-scan. This arrangement effectively prevents deterioration of the picture quality caused by the combination of the picture quality-deteriorating factor arising at each joint of adjoining nozzle units due to the individual difference between the nozzle units with the picture quality-deteriorating factor arising at the rear most end of the head assembly due to the error in feeding amount of sub-scan of the nozzle unit.
In accordance with one preferable application of the present invention, the printing apparatus has the head assembly where the joints of the respective nozzle units included therein satisfy the following conditions, while setting the amount of sub-scan equal to N raster lines that corresponds to the number of effective raster lines. The number of effective raster lines represents the net number of raster lines formed by one pass of the main scan. For example, each raster line is formed by two passes of the main scan. In general, it is the that Nxe2x80x2 raster lines are formed by s passes of the main scan. In this general case, it is thought that each pass of the main scan forms Nxe2x80x2/s raster lines. Namely the number of effective raster lines is equal to Nxe2x80x2/s. The position of the connection of any adjoining nozzle units is adjusted to make the joint of the nozzle units apart by at least k raster lines from a specified position. The specified position here is apart by an integral multiple of the number of effective raster lines, which is equal to N, from the rear most nozzle in each nozzle unit included in the head assembly. The amount of sub-scan is set equal to N raster lines, which corresponds to the number of effective raster lines, since this condition is required to complete the interlace as discussed later.
When a fixed value (corresponding to N raster lines) is set to the amount of sub-scan, every pass of the sub-scan shifts the rear most nozzle in each nozzle unit by N raster lines. The sub-scan that makes the position of the rear most nozzle apart by at least k raster lines from the position of the joint of the adjoining nozzle units effectively prevents any overlap of the printing positions corresponding to the joints of the adjoining nozzle units to worsen the picture quality. In a similar manner, this arrangement also effectively prevents any overlap of the printing positions corresponding to the rear most end of the head assembly and the joint of the adjoining nozzle units. This arrangement accordingly prevents deterioration of the picture quality caused by the combination of the picture quality-deteriorating factor due to the error in feeding amount of sub-scan with the picture quality-deteriorating factor due to the individual difference between the nozzle units.
Even in the case where each raster line is completed by one pass of the main scan, the number of effective raster lines N on the head assembly is not always coincident with the total number of nozzles. One modified application appropriately selects effective nozzles among all the nozzles provided on the head assembly to attain a predetermined nozzle pitch and uses only the selected nozzles to eject ink droplets.
In accordance with one preferable application of the present invention, the printing apparatus has the head assembly of the following configuration, while setting the amount of sub-scan equal to N raster lines that corresponds to the number of effective raster lines. The head assembly forms raster lines at equal intervals of a k raster lines space by each pass of the main scan. Each of the nozzle units included in the head assembly forms n raster lines by each pass of the main scan. In order to complete the interlace, N and k should be prime to each other as described later. The head assembly should also be designed to satisfy the relationship of (n/s)xe2x89xa7k. Selection of N, k, n, s satisfying such conditions effectively prevents the printing position corresponding to the joint of any pair of adjoining nozzle units or the rear most end of the head assembly from overlapping with the printing position corresponding to the joint of another pair of adjoining nozzle units. Here the condition that N and k are prime to each other means that the greatest common divisor of the two integers N and k is equal to 1.
The reason why the arrangement of fulfilling the above conditions prevents any overlap of the printing position corresponding to the joint of each pair of adjoining nozzle units or the rear most end of the head assembly with the printing position corresponding to the joint of another pair of adjoining nozzle units will be discussed later, since the reason is also related to the conditions to complete the interlace.
The printing apparatus of the present invention may use the head assembly having the following configuration and satisfying the conditions specified below for the amount of sub-scan. The head assembly forms raster lines at equal intervals of a k raster lines space in each pass of the main scan, where each nozzle unit forms n raster lines. In this application, a set of feeding amounts of sub-scan, which include a plurality of different feeding amounts, are stored in advance. The set of feeding amounts of sub-scan are defined by that the absolute value of a difference between each cumulative value, which is obtained by successively accumulating the feeding amounts of sub-scan, and an integral multiple of nxc3x97k is not less than k.
The successive sub-scan of the head assembly according to the set of feeding amounts of sub-scan thus stored causes the joint of each pair of adjoining nozzle units to be shifted by the cumulative value of the feeding amounts of sub-scan. Each nozzle unit forms n raster lines at the equal intervals of the k raster lines space. The joints of the nozzle units are accordingly present at an interval of an nxc3x97k raster lines space. When the absolute value of the difference between the cumulative value of the feeding amounts of sub-scan and the integral multiple of nxc3x97k is less than k, there are overlaps of the printing positions corresponding to the joints of the nozzle units. The arrangement of selecting and storing the set of feeding amounts of sub-scan to make the absolute value of the difference not less than k thus desirably prevents any overlap of the printing positions corresponding to the joints of the nozzle units in the course of the sub-scan of the head assembly.
One applicable procedure first specifies the configuration of the head assembly, such as the interval between adjoining raster lines and the number of raster lines formed in each pass of the main scan and the structure of the nozzle units included in the head assembly, and then selects the amount of sub-scan to fulfill the respective conditions discussed above, based on the specified configuration of the head assembly. Another applicable procedure, on the contrary, first selects the amount of sub-scan and then determines the configuration of the head assembly and the positions of the joints of the adjoining nozzle units.
The technique of the present invention may be attained by carrying out sub-scan and main scan of a head assembly according to the method discussed above in a computer system, which includes a printing apparatus that has the head assembly obtained by combining a plurality of nozzle units, and a computer that controls the printing apparatus. Namely the present invention may be constructed as a recording medium in which a program for actualizing the method is recorded in a computer readable manner.
The present invention is thus directed to a recording medium in which a program is recorded in a computer readable manner to actualize a method of printing an image by causing ink droplets to be ejected from a nozzle array, which is formed by arranging a plurality of nozzles, to create ink dots on a printing medium while changing a relative position of the nozzle array to the printing medium. The program causes a computer to attain the functions of: creating ink dots while successively changing the relative position of the nozzle array to the printing medium in a direction that crosses an extending direction of the nozzle array, so as to form raster lines on the printing medium at intervals of a k raster lines space, where each raster line represents an array of dots; and successively changing the relative position of the nozzle array to the printing medium in the extending direction of the nozzle array by a predetermined amount, so as to record all raster lines included in an effective area on the printing medium, each change of the relative position causing a rear most nozzle included in each nozzle unit among plural nozzle units to be located at a specific position that is apart by at least k raster lines from a certain position where a rear most nozzle included in another nozzle unit among the plural nozzle units is located prior to the change of the relative position, where each nozzle unit has the plurality of nozzles.
The computer reads the program to attain such functions and controls the main scan and the sub-scan of the head assembly including the plurality of nozzle units. This arrangement also effectively prevents the picture quality from being worsened by any overlap of the printing positions, for example, corresponding to the joints of the nozzle units.
In accordance with one preferable application of the printing apparatus of the present invention discussed above, a plurality of head assemblies are arranged in the main scanning direction to satisfy the following positional relationship. Each nozzle unit included in the head assembly may have the ejection characteristic of the ink droplets that periodically varies in the sub-scanning direction, due to the structure of the nozzle unit. The plurality of head assemblies are thus arranged to be shifted in position in the sub-scanning direction, such that the periodic variations in ejection characteristic of the ink droplets in the respective head assemblies do not overlap one another.
In one concrete embodiment, the respective head assemblies may be arranged to prevent their periodic variations in at least one of ejection speed of the ink droplets and ejection amount of the ink droplets from overlapping one another.
There are a variety of reasons for the periodic variation in ejection characteristic of the ink droplets. For example, in a nozzle array consisting of a large number of nozzles provided on the nozzle unit, each end portion of the nozzle array has three sides fixed by the walls of the nozzle unit, whereas the central portion of the nozzle array has only two sides fixed by the walls of the nozzle unit. This leads to a variation in rigidity of the nozzle unit. In the event that a reinforcement is attached to the central portion of the nozzle array to supply the additional rigidity, the end portions and the central portion of the nozzle unit have the higher rigidity, while the other portions of the nozzle unit have relatively lower rigidity.
The variation in ejection characteristic of the ink droplets may be ascribed to variations of the ink supply system and the electrical system as well as to the variation in rigidity of the nozzle unit. In the case of the long nozzle array, the nozzle array may be divided into a plurality of blocks, and the supply of ink and the supply of electric power for driving the nozzles may be regulated in each block. In this case, the division into the plural blocks may cause the periodic variation in ejection characteristic of the ink droplets. The variation in ink ejection characteristic generally worsens the printing quality.
In the application of the present invention, however, a head assembly group is obtained by combining a plurality of head assemblies in the main scanning direction. The respective head assemblies are arranged to prevent their periodic variations in ejection characteristic of the ink droplets from overlapping one another. This arrangement desirably suppresses the total periodic variation in the group of the head assemblies, thus improving the printing quality.
FIG. 19 conceptually shows an arrangement of plural nozzle units in such a manner that their periodic variations in ink ejection characteristic are successively shifted and thereby cancelled out. The left side of the drawing shows an arrangement of two nozzle units, and the right side of the drawing shows ink dots respectively created by the two nozzle units. As clearly understood from the ink dots created by the respective nozzle units, in the example of FIG. 19, each nozzle unit has the variation of three periods. The arrangement of mutually shifting the period of the variation in each nozzle unit averages the variations in ejection characteristic of the ink droplets. Especially the arrangement of shifting the variation by half the period causes the antinodes of the periodic variations to overlap the nodes of the periodic variations, thus most effectively averaging the variations in ink ejection characteristic. The parts of the two nozzle units that do not overlap each other in the main scanning direction may or may not contribute to creation of ink dots.
In accordance with one embodiment of the printing apparatus having the head assembly group discussed above, the plurality of head assemblies included in the head assembly group may be arranged, such that nozzle positions in the respective head assemblies are shifted in the main scanning direction by approximately half a nozzle pitch. This arrangement averages the variations in ink ejection characteristic of the nozzle units, while attaining a small nozzle pitch in the head assembly group. This arrangement is discussed with reference to FIG. 20. FIG. 20 shows an arrangement of a head assembly group that includes two head assemblies combined with each other in the main scanning direction, where each head assembly includes two nozzle units combined with each other in the sub-scanning direction. A variation in ink ejection characteristic of one period is assumed here, where the ink ejection speed or the amount of ink ejection decreases in both end portions of each nozzle unit and increase in the central portion. The arrangement of the two head assemblies to prevent their periodic variations of the nozzle units from overlapping each other desirably averages the variations in ink ejection characteristic. This arrangement also causes the nozzles of the two head assemblies to be positioned alternately, thus giving a nozzle array having a small nozzle pitch.
In accordance with another embodiment of the printing apparatus having the head assembly group discussed above, the plurality of head assemblies included in the head assembly group may be arranged, such that nozzle positions in the respective head assemblies are identical in the main scanning direction. This arrangement also averages the variations in ink ejection characteristic of the nozzle units. In this case, ink droplets may be ejected simultaneously from one nozzle unit as shown in FIG. 21 or may be ejected alternately from the two nozzle units as shown in FIG. 22. In either case, the arrangement of making the nozzle positions coincident with each other in the main scanning direction enables each raster line to be formed with two nozzles. This makes the subtle variation in ink ejection characteristic due to the individual difference between the nozzle units significantly inconspicuous, thus improving the printing quality.